Voltage mode differential driver and method

ABSTRACT

A differential driver includes a switching module and first and second voltage controlled voltage sources. The switching module has a plurality of switches each controlled by an input signal, a first voltage input and a second voltage input, and a signal output. The first voltage controlled voltage source is connected to the first voltage input. The first voltage controlled voltage source has a low impedance. The second voltage controlled voltage source is connected to the second voltage input. The second voltage controlled voltage source also has a low impedance. The switching circuit outputs an output signal having an output voltage and current controlled by the first and second voltage controlled voltage sources. The output signal is based upon the input signal.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. provisional application Ser.No. 60/331,520, filed Nov. 19, 2001, the contents of which is herebyincorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to input/output (I/O) interface circuitry for highspeed data communications applications. More specifically the inventionrelates to low voltage differential signaling (LVDS) drivers, for use inthe fields of communications, video and other integrated circuits thatdemand very high data transfer rates.

2. Description of the Related Art

Differential drivers are well known. Differential drivers are used inmany input/output (I/O) applications such as in communications, videoand integrated circuits that may demand high data transfer rate.Differential drivers are used in integrated circuits (IC) for on-chipcommunications between circuits, chip-to-board, off-chip communications,etc.

Low-voltage differential signaling (LVDS) technology was developed inorder to provide a low-power and low-voltage alternative to otherhigh-speed I/O interfaces specifically for point-to-point transmissions,such as those used in a network devices within data and communicationnetworks. LVDS drivers can be implemented to overcome some deficiencieswith previous I/O interface circuitry. However, the LVDS standardprovides strict specifications for signal input and outputcharacteristics, such as common mode voltage, differential voltage, etc.

In conventional I/O designs, high-speed data rates are accomplished withparallel I/O structures, each I/O device typically having a limitedbandwidth. As bandwidth increases, more I/O devices are required toachieve the increased bandwidth. Over the years, bandwidth has increasedsubstantially leading to massive parallelism in I/O designs in ICs. As aresult, these parallel I/O structures occupy more and more space on ICs.This complicates the design of the circuits because there is lessavailable space on the chip. The use of parallel structures also createsa need for additional supporting power supplies because of the numerousextra pads, current sources, etc. necessary in a parallel structure.Thus, most existing I/O drivers are not power efficient.

In portable devices, such as laptop computers, the power coming from thebattery, low power allows for longer operating time. In the case wherepower is not restricted, such as in a desk top PC, power consumption isalso important in IC. For example, if a CPU consumes more power, it willrequire an expensive package for the IC and possibly an additionalcooling fan. Therefore, lower power means lower cost to the system.

A prior art LVDS driver is shown in FIG. 1. The metal oxide silicon(MOS) transistor 100 is represented with a circle at the gate indicatingthat it is a P-type MOS (PMOS) transistor. Transistors 101, 110, 111,120 and 121 are N-type (NMOS) transistors. The driver includes twocurrent sources 100 and 101, and four current switching NMOS transistors110, 111, 120, and 121. PMOS transistor 100 provides current from VDD tothe top switching transistors 110 and 121. A bias voltage Vb1 controlsthe amount of current following through the transistor 100. The bottomNMOS sinks current from the switching transistors 120 and 111 to ground(GND). A second bias, voltage Vb2, controls the current followingthrough the transistor 101. Biasing this circuit is fairly easy, andbias voltages are typically provided using current mirrors.

In normal operation, only one group of switching can be on. In the casewhen transistors 110 and 111 are ON and 120 and 121 are OFF, the currentfrom the current source 100 flows through the switching transistor 100and follows to the load resistor 130. A voltage drop develops on theterminal of the resistor 130. Since, in this case, the current followsfrom bottom node 132 to top node 131, the bottom node 132 has a higherpotential than the up node 131. The current on the top node 131 is sunkby current source 101 through the switching transistor 111. The currentsource 101 should sink the same amount of current as provided by currentsource 100, to get the common mode voltage correctly.

In the opposite case, when transistors 110 and 111 are OFF andtransistors 121 and 121 are ON, current will create a voltage drop of areversed polarity on the load resistor 130. In this case, the top node131 has a higher potential than the bottom node 132.

There are two major drawbacks in this circuitry for high speed ICapplications. First, operating speed is limited due to the highimpedance design. Node Vhigh and node Vlow are high impedance nodes withrelatively large parasitic capacitance, and therefore, are slow torespond. In high speed switching, these nodes also cause the common modevoltage to drift. A poorly designed current source, as an example, couldhave an impedance above a few kilo-ohms. Moreover, a well designedcurrent source will have much higher impedance. Moreover, a welldesigned current source, such as cascoded current source, will have muchhigh impedance.

Second, in a high speed serial interconnection, termination at thedriver side may be required for good signal integrity. This circuit doesnot include terminal resistors, and therefore, has poor signal integrityat high speeds.

FIG. 2 shows another prior art implementation of an LVDS driver that hasbuilt-in termination resistors. The operation of the circuit is verysimilar to the first circuit, except the load is now shared with theresistors 150 and 151. The impedances at the current source 100 and 101are very high and can be neglected compared to the termination resistor.To terminate the source properly, resistors 150 and 151 need to be halfthe resistance of the resistor 130. For a typical application, resistor130 is 100 ohms. Thus, resistors 150 and 151 need to be 50 ohms each. Inthis design, the same amount of current will follow into resistors 150and 151. The advantage of adding resistors 150 and 151 is that theimpedance at Vhigh and Vlow are reduced for high speed operation. Also,since this reduces reflection in the transmission line, signal integrityis improved. However, the current efficiency of this driver is 50%because only 50% of the current generated flows to the load. Thus, thiscircuit design is deficient for having a low current efficiency.

In view of the deficiencies in the prior art, there is a need for newand improved systems and methods for driving LVDS in modern I/Oapplications.

SUMMARY OF THE INVENTION

According to an embodiment of the present invention, a differentialdriver is provided. The differential driver includes a switching moduleand first and second voltage controlled voltage sources. The switchingmodule has a plurality of switches each controlled by an input signal, afirst voltage input and a second voltage input, and a signal output. Thefirst voltage controlled voltage source is connected to the firstvoltage input. The first voltage controlled voltage source has a lowimpedance. The second voltage controlled voltage source is connected tothe second voltage input. The second voltage controlled voltage sourcealso has a low impedance. The switching circuit outputs an output signalhaving an output voltage and current controlled by the first and secondvoltage controlled voltage sources. The output signal is based upon theinput signal.

According to another embodiment of the present invention, a method ofdriving a signal is provided. The method includes a step of providing aswitching module having a first and second voltage input, a signalinput, and a signal output. The signal input is connected to a pluralityof switches in order to control an operation of the switches. The signaloutput is connected to the first and second voltage inputs via theplurality of switches. The method also includes a step of providing afirst voltage controlled voltage source having a first voltage outputhaving a low impedance. The method also includes a step of providing asecond voltage controlled voltage source having a second voltage outputhaving a low impedance. The method also includes a step of connectingthe voltage output of the first voltage controlled voltage source to thefirst voltage input of the switching module. The method also includes astep of connecting the voltage output of the second voltage controlledvoltage source to the second voltage input of said switching module.

According to another embodiment of the present invention, a differentialdriver is provided. The differential driver includes a switching meansand first and second voltage controlled voltage source means. Theswitching means is for switching a plurality of switches in order toproduce a signal output based on an input signal, a first and secondvoltage input. The first voltage controlled voltage source means is forgenerating a first low impedance voltage output as the first voltageinput to the switching means. The second voltage controlled voltagesource means is for generating a second low impedance voltage output asthe second voltage input to the switching means.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and features of the invention will be more readilyunderstood with reference to the following description and the attacheddrawings, wherein:

FIG. 1 is an illustration of a prior art LVDS driver;

FIG. 2 is an illustration of a prior art LVDS driver having terminalresistors;

FIG. 3 is an illustration of a LVDS driver according to an embodiment ofthe present invention;

FIG. 4 is an illustration of a voltage mode differential driveraccording to another embodiment of the present invention;

FIG. 5 is a flowchart of a method for driving a signal according to thepresent invention;

FIG. 6 is an illustration of a voltage mode differential driveraccording to an additional embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 3 is an illustration of a voltage mode differential driveraccording to an embodiment of the present invention. The differentialdriver includes two Voltage Controlled Voltage Sources (VCVS) 210 and211, which provide DC voltages to the nodes Vhigh and Vlow,respectively. The output impedance for voltage controlled voltage source210 is modeled by a resistor 202 and is configured to be a low impedancein the range of a few hundred ohms, preferable around 30 ohms for adifferential load of 100 ohms. Similarly, the output impedance for thevoltage controlled voltage source 211 is modeled by resistor 203 and issimilarly configured to be a low impedance. VCVS 210 and 211 are biasedby bias voltages Vb1 and Vb2, respectively output by a bias generator215.

The differential driver also includes a switching circuit having aplurality of switches for switching an output voltage (signal) based onan input signal. The switching circuit may include a pair of voltageinputs, at nodes Vhigh and Vlow, and series of switches. In thisexample, the switching circuit includes two pair of switchingtransistors 110 a, 111 a and 120 a, 121 a, which act as the switches. Asignal output, Vout1 and Vout2, are output from the switching circuit tonodes 131 and 132 across a resistive load 130. Load 130 may typically be100 ohms, but can vary depending on the application. Load 130 may be adifferential load, and accordingly, may be grounded in the middle of theload. As a differential load, load 130 would include 50 ohms above theground and 50 ohms below the ground. Output node 131 is connected to thedrains of transistors 110 a and 120 a, and output node 132 is connectedto the drains of transistors 121 a and 111 a.

In normal operation, only a single pair of switching transistors will beON while the other is OFF, in order to allow a current to flow from VCVS210 through the load 130 to VCVS 211. In the present embodiment,switching transistors 110 a and 121 a are PMOS transistors, andswitching transistors 120 a and 111 a are NMOS transistors. PMOStransistors are used on the top of the switching circuit because thecommon mode voltage is around half of the supply voltage, or about 1.2V.The PMOS transistors could have a smaller size than that of NMOStransistors. If NMOS were used, the size would be very bigcomparatively, because of the Vgs required to turn the switchingtransistor completely ON. If NMOS transistors were used, then there is arisk of common mode voltage drifting, which will place the common modevoltage outside of the LVDS standard for LVDS applications.

An input signal is used to control the switching of the switchingcircuit. In order to turn the pair 110 a, 111 a ON and pair 120 a, 121 aOFF simultaneously, input signal IN+ is sent to transistor 110 a whilean inverted signal IN− is sent to transistor 111 a to turn bothtransistors ON. Similarly, IN+ is sent to transistor 120 a whileinverted signal IN− is sent to transistor 121 a to turn them both OFF.Therefore, the gates of transistors 110 a and 120 a may be connected,while the gates of 121 a and 111 a may be connected. In a preferredembodiment of the present invention, the differential driver is used inLVDS applications. In LVDS applications, IN+ may be 2.5V while IN− maybe 0V.

The voltage drop across node Vhigh and Vlow is calculated by:

Vhigh−Vlow=I*(R(110 a)+R(111 a)+Rload),

where R(110 a) and R(111 a) are the ON resistance of transistors 110 aand 111 a, and I is the current required to be flowing through the loadresistor. Of course, the current I at the load may be set to comply withthe LVDS standard. The biasing and configuration of the VCVS 210 and 211may likewise be adjusted in accordance with the LVDS standard or toobtain a desired output Vout2−Vout2. Accordingly, the voltage suppliedat Vhigh and Vlow are calculated to provide the proper common modevoltage and current output to the load. Because this circuit has verylow impedance at Vlow, and Vhigh, it is capable of high speed operationwith high efficiency.

The switching circuit may also provides gain, and the differentialdriver may act as an amplifier to amplify the incoming signal. Asdescribed above, the gates of transistors 110 a and 120 a, hereafterreferred to as GO, may be tied together, and the gates of transistors111 a and 121 a, hereafter referred to as G1, may be tied together. Thegain may be calculated by:

A(v/v)=[V(131)−V(132)]/[V(G0)−V(G1)]=[gm(110 a)+gm(120 a)]*R(130),

where gm(110 a) and gm(120 a) are the transconductance of transistors110 a and 120 a. As an example, when there is a current of around 3 mAflowing into the load resistor 130, then gm(110 a) or gm(120 a) istypically around 15 millisiemens (mS). Thus,

A(v/v)=(15 mS+15 mS)*100 ohms=3 V/V.

Here is an example when the incoming signal is 50 mV and the outputsignal would be 150 mV. Note that this is a small signal gain. In apreferred embodiment, the incoming signals are large signal (as opposedto small signal). The voltage swing across G0 and G1 may be 0 to 2.5V.Therefore, the output would be 7.5V if the output is not limited by thepower supply. The results are that these four transistors are working intriode region (with gm much smaller than 15 mS), where the resistancebetween source and drain may be around 10 ohms. When the transistors areOFF, the resistance across drain and source are infinity, and when thetransistors are ON, the resistance across the drain and source is onlyabout 10 ohms. Therefore, the transistors may be referred to or replacedby switches.

An advantage of the embodiment illustrated in FIG. 3 is that it allowsfor low power consumption with high speed operation through the usesource followers at nodes Vhigh and Vlow. The impedance looking into thesource follower is significantly lower than if a current source wereused. Thus, the nodes Vhigh and Vlow can be operated at high speed.

FIG. 4 is an illustration of voltage mode differential driver accordingto another embodiment of the present invention. The differential driverincludes VCVS 210 and VCVS 211, which provide DC voltages to the nodesVhigh and Vlow, respectively, and a switching circuit.

VCVS 210 includes a transistor 300, which is implemented by an NMOStransistor in a source follower configuration. Similarly, VCVS 211includes transistor 301, which is implemented by a PMOS transistor in asource follower configuration. Source followers provide significantlylower impedance looking into the source follower, i.e., at nodes Vhighand Vlow, than that of a current source, such as shown in FIGS. 1-2. Forexample, in an LVDS application, a typical driver current I flowing tothe load is 3 mA. When a 3 mA current flows in the transistors 300 or301, the impedance looking into source is very low, and can be wellbelow 50 ohms. Resistors 190 and 191 may be added to VCVS 210 and 211 toprotect the differential driver from excessive current in the case ofshorted terminals.

The switching circuit includes a pair of voltage inputs, connected atnodes Vhigh and Vlow, and two pair of switching transistors 110 a, 111 aand 121 a, 122 a, which act as the switches. In the present embodiment,switching transistors 110 a and 121 a are PMOS transistors, andswitching transistors 120 a and 111 a are NMOS transistors. PMOStransistors are used on the top of the switching circuit because thecommon mode voltage is around half of the supply voltage, which is about1.2V. The PMOS transistors could have a smaller size than that of NMOS.If NMOS were used, the size would be very big comparatively, because ofthe Vgs required to turn the switching transistor completely ON.Moreover, if NMOS transistors were used, then there is a risk of commonmode voltage drifting, which will place the common mode voltage outsideof the LVDS standard for LVDS applications. Vout1 and Vout2 are outputfrom the switching circuit to nodes 131 and 132 across a resistive load130, which may be a differential load as described above. Load 130 maytypically be 100 ohms, such as in LVDS applications. Output node 131 isconnected to the drains of transistors 110 a and 120 a, and output node132 is connected to the drains of transistors 121 a and 111 a.

The source of transistor 300 is connected to node Vhigh, which providesvoltage to the top of switching transistors 110 a and 121 a. The gate oftransistor 300 is connected to bias voltage Vb1, the drain is connectedto VDD through resistor 190, and the P-well is also connected nodeVhigh. In normal process the substrate (P-well) is connected to groundby default. However, it is preferred that the substrate be connected tothe source (node Vhigh) to reduce the body effect and lower thethreshold voltage of the transistor. The lowering of the thresholdvoltage allows the MOSFET to be smaller for the same amount of current.Therefore, the area of the IC can be smaller and cost can be lower.

From gate to source, transistor 300 can have a voltage gain of 0.8V/V-1.0 V/V, and also will have some DC level shifting (Vt+Vdsat in thiscase). Transistor 300 has a low impedance given by 1/gm, where gm is thetransconductance of the source follower 300. The low impedance at nodeVhigh allows faster response time at the node, and therefore, allows forbetter high-speed switching output from the differential driver at load130.

Transistor 301 may be a PMOS transistor. The source of transistor 301 isconnected to Vlow and provides solid low impedance voltage for theswitching transistors 120 a and 111 a. The gate of transistor 301 isconnected to bias voltage Vb2, the drain is connected to ground (GND)through resistor 191, and the N-well is preferably connected to source(node Vlow). Connecting the N-well to the source lowers the thresholdvoltage and provides more “headroom” for operating at low supplyvoltage.

Similar to that described above with reference to FIG. 3, in normaloperation, only one pair of switching transistors are switched ON at atime. An input signal is used to control the switching of the switchingcircuit. In order to turn the pair 110 a, 111 a ON and pair 120 a, 121 aOFF simultaneously, input signal IN+ is sent to transistor 110 a whilean inverted signal IN− is sent to transistor 111 a to turn bothtransistors ON. Similarly, IN+ is sent to transistor 120 a whileinverted signal IN− is sent to transistor 121 a to turn them both OFF.Therefore, the gates of transistors 110 a and 120 a may be connected,while the gates of 121 a and 111 a may be connected. In a preferredembodiment of the present invention, the differential driver is used inLVDS applications. In LVDS applications, IN+ may be 2.5V while IN− maybe 0V.

In the case where transistors 110 a and 111 a are ON and 121 a and 120 aare OFF, the transistor 300 provides a voltage Vhigh to drive a currentthrough the MOS switch 110 a to the load resistor 130, then throughtransistor 111 a to the source of transistor 301. Similarly, when 110 aand 111 a are OFF and 121 a and 120 a are ON, the transistor 300provides a voltage Vhigh to drive current through the MOS switch 121 ato the load resistor 130, then through transistor 120 a to the source oftransistor 301.

Using source followers to provide voltage to the switching circuitrequires proper biasing, especially in a low voltage or LVDSapplications. The bias voltage Vb1 may be determined as follows:

Vb 1=(Vhigh+Vtn+Vdsat),

where Vtn is the threshold voltage of the NMOS transistor 300, and Vdsatis the overdrive voltage for the NMOS transistor 300 when conducting acertain amount of current. Since the current flows from bottom node 132to top node 131, the bottom node 132 has a higher potential than the upnode 131. The PMOS transistor 301 provides a low voltage at node Vlow tosink current from the load resistor 130 through the transistor 111. Thevoltage drop at the load resistor (130), the required voltage differenceof Vhigh and Vlow can be calculated as followed:

Vhigh−Vlow=I*[R(121 a)+R(130)+R(120 a)],

where R(121 a), R(130) and R(120 a) are the ON resistance of transistors121 a, 130 and 120 a respectively. Transistors 110 a and 121 a may beprovided to have identical sizes, and so may transistors 120 a and 111a, and are preferably 20 ohms for a differential load of 100 ohms. Thus,the common mode voltage is calculated by

(Vhigh+Vlow)/2,

since the resistance of transistors 121 a and 120 a are designed to bethe same ideally. On the opposite case, when transistor 110 a and 111 aare OFF and transistor 121 a and 120 a are ON, the voltage drop on theload resistor 130 will be reversed polarity. The top node 131 has ahigher potential than the bottom node 132.

Similar calculations can be made to determine the necessary bias voltageVb2. In low voltage applications it may be desired to provide at leastone voltage source greater than 1.2 volts in order to ensure that properbiasing of the circuit is obtained.

Similar to above, the driver of this embodiment may provide gain.Accordingly, the switching transistors and the power supplies may beconfigured to apply a small signal or large signal gain to the incomingsignal. Furthermore, the switching transistors may be replaced byswitches.

There are two major advantages in this implementation. First, asdescribed above, node Vhigh and node Vlow are very low impedance nodes.Although the nodes have relatively large parasitic capacitance, they arefast to respond. Therefore, the differential driver is capable of highspeed operations. Second, the driver may include built-in terminatedresistance, for better signal integrity. To terminate the differentialdriver properly, the impedance of the driver needs to be the same as thetransmission line. A typical transmission line has single ended 50 ohmsimpedance, thus the output impedance should be 50 ohms. Take the examplewhen transistors 110 a and 111 a are closed. The MOS transistors 110 aand 111 a have impedances of R(110 a) and R(111 a), respectively. Theimpedance looking into source of the NMOS transistor 300 is 1/gm (300).Thus, to get a total impedance of 50 ohms, one should design R(110a)=50−1/gm (300) ohms. The same can be said for the PMOS side,transistor 301, and one should design R(111 a)=50−1/gm (301) ohms.Therefore, if the load 130 is a differential load of 100 ohms, half theload (50 ohms) is mirrored by the top half of the driver (R(300)+R(110 aor 121 a)=50 ohms) and the other half of the load is mirrored by thebottom half of the driver (R(301)+R(111 a or 120 a)=50 ohms).

Because of the built-in termination resistance, the circuit does notneed additional termination resistors in parallel with the load.Therefore the circuit in FIG. 4 can achieve 100% current efficiency,without wasting current in the passive termination resistors.

It should be noted that a more linearized output impedance may beprovided by added a linear resistor between the source of each VCVS andthe node Vhigh and Vlow, respectively. Accordingly, FIG. 6 shows linearresistors 400 and 401 added to VCVS 210 and 211, respectively. In thiscase, when 1/gm is small, the linear resistors can be added to get anoutput impendence of R(301)+R(111 a or 120 a)+R(400), and R(300)+R(110 aor 121 a)+R(401).

FIG. 5 is a flowchart of a method for driving a signal according to anembodiment of the present invention. The process begins at step S5-1. Atstep S5-2, a switching module is provided, such as described above. Theswitching module may be implemented via MOS transistors. The switchingmodule may include a voltage input and output, and a signal input andoutput. The voltage input is connected to the switches in order to flowa current to the signal output. The switching module may be configuredto receive an input signal and switch the switches to produce a signaloutput based on the signal input. The switching module may be configuredfor LVDS applications and may have built-in termination resistors asdescribed above.

Next, at step S5-3, low impedance, voltage controlled voltage sourcesare provided at the voltage input and output of the switching module.Voltage controlled voltage sources may be as already described above andmay include source followers.

Next, at step S5-4, the voltage controlled voltage sources are biasedfor the application of the driver. The biasing of the voltage controlledvoltage sources can be done by a bias generator or other circuit, andmay be implemented in accordance with the above-described embodiments.For LVDS applications, the biasing of the circuit should take intoconsideration the desired output voltage and current of the switchingmodule, as well as all the characteristics of the source followers andthe switches themselves.

Next, at step S5-5, an input signal may be provided. The input signalmay be input via an input circuit to each switch, as already describedabove. Depending upon the configuration of the switches, the inputsignal may be inverted, pulled-up or pulled-down. The voltage controlledvoltage source and the switching module are configured, as describedabove, to generate a high speed output signal based up the input signal.This output signal may be in compliance with LVDS standards.

One having ordinary skill in the art will understand that these methodsteps may be performed in different orders to accomplish the sameresult.

Embodiments of the present invention may be drawn to differentialdrivers such as LVDS drivers that can operate at high speed with lesspower because it operates with a reduced voltage swing. Due to thereduced voltage swing, which allows the LVDS driver to operate at highspeeds, less parallelism is needed. Also with the differential outputs,a receiver can reject ambient common mode noise and system reflectionnoise. However, performance can vary significantly for LVDS drivers ofdifferent designs. Two important parameters to consider are operationfrequency and power consumption.

Although the invention has been described based upon these preferredembodiments, it would be apparent to those of skilled in the art thatcertain modifications, variations, and alternative constructions wouldbe apparent, while remaining within the spirit and scope of theinvention. In order to determine the metes and bounds of the invention,therefore, reference should be made to the appended claims.

For example, VCVS 210 and 211 may include an operational amplifier, andthe impedance of the drive my be controlled by adding an extratermination resistor Rtt. In this case, the loop gain of the opamp andof transistor 300 or 301, can be large, and the impedance can be verylow. In this case, the output impedance is dominated by Rtt, and Rtt maybe set close to 50 ohms to get good termination.

Furthermore, other active devices, such as BJTs or BiCMOS transistorsmay be used. In this case, transistor 300 could be an NPN transistorwith the emitter connected to Vhigh, and transistor 301 may be a PNPtransistor with the emitter connected to Vlow.

We claim:
 1. A differential driver comprising: a switching module havinga plurality of switches each controlled by an input signal, a firstvoltage input and a second voltage input, and a signal output; a firstvoltage controlled voltage source connected to said first voltage input,said first voltage controlled voltage source having a low impedance; anda second voltage controlled voltage source connected to said secondvoltage input, said second voltage controlled voltage source having alow impedance; wherein said switching circuit outputs an output signalhaving an output voltage and current controlled by said first and secondvoltage controlled voltage sources, said output signal being based uponsaid input signal and wherein said first voltage controlled voltagesource comprises a first source follower circuit and said second voltagecontrolled voltage source comprises a second source follower circuit. 2.The differential driver as recited in claim 1, further comprising: abias generator connected to a first bias input of said first voltagecontrolled voltage source and a second bias input of said second voltagecontrolled voltage source, said bias generator outputting a first biasvoltage to said first bias input and a second bias voltage to saidsecond bias input, said first and second bias voltage adjusting saidoutput voltage and current controlled by said first and second voltagecontrolled voltage sources.
 3. The differential driver as recited inclaim 2, wherein said first bias voltage and said second bias voltageare set so that said output voltage and current are within a LVDS range.4. The differential driver as recited in claim 1, wherein at least oneswitch of said plurality of switches is turned ON by said input signalinput and at least one switch of said plurality of switches is turnedOFF by said input signal, said at least one switch of said plurality ofswitches being turned ON connecting said first voltage input with saidsignal output.
 5. The differential driver as recited in claim 1, whereinsaid first source follower circuit includes a first source followertransistor in a source follower configuration and a first protectiveresistor between a first voltage supply and a drain of said first sourcefollower transistor, and said second source follower circuit includes asecond source follower transistor in a source follower configuration anda second protective resistor between a second voltage supply and a drainof said second source follower transistor.
 6. The differential driver asrecited in claim 1, wherein said plurality of switches comprise aplurality of transistors, at least one first transistor receiving saidinput signal at a gate thereof, switching said first transistor ON, andat least one second transistor receiving said input signal invertedswitching said second transistor OFF.
 7. The differential driver asrecited in claim 1, wherein said switching module comprises a first andsecond pair of transistors, a first transistor of said first pair havinga source connected to a source of a first transistor of a second pair, asecond transistor of said first pair having a drain connected to a drainof said first transistor of said first pair, a second transistor of saidsecond pair having a drain connected to a drain of said first transistorof said first pair, said first transistor of said first pair having adrain connected to said first voltage input, said first transistor ofsaid second pair having a drain connected to said first voltage input,said second transistor of said first pair having a source connected tosaid second voltage input, said second transistor of said second pairhaving a source connected to said second voltage input, said firsttransistor of said first pair and said second transistor of said secondpair each having a gate connected to a first signal input receiving saidinput signal, said first transistor of said second pair and said secondtransistor of said first pair each having a gate connected to a secondsignal input receiving said input signal inverted, a first signal outputis connected to the drain of said second transistor, a second signaloutput is connected to the drain of said first transistor, said firstpair of transistors being ON and said second pair of transistors beingOFF when said input signal is a positive signal, and said first pair oftransistors being OFF and said second pair of transistors being ON whensaid input signal is a negative signal.
 8. The differential driver asrecited in claim 7, wherein said first transistor of said first pair oftransistors and said first transistor of said second pair of transistorsare a first type of transistor, said second transistor of said firstpair of transistors and said second transistor of said second pair oftransistors are a second type of transistor.
 9. The differential driveras recited in claim 8, wherein said first type of transistor is a P-typetransistor and said second type of transistor is an N-type transistor.10. The differential driver as recited in claim 7, wherein said firstpair of transistors have a resistivity which is substantially equal toone another, and said second pair of transistors have a resistivitywhich is substantially equal to one another.
 11. The differential driveras recited in claim 10, wherein said first pair of transistors have atotal resistivity which is substantially equal to a resistivity of aload across said first and second signal outputs, and said second pairof transistors have a total resistivity which is substantially equal tosaid resistivity of said load.
 12. The differential driver as recited inclaim 10, wherein the resistivity of said first pair of transistors whenadded to the resistivity of said first and second voltage controlledvoltage sources which is substantially equal to a resistivity of a loadacross said first and second signal outputs, and the resistivity of saidsecond pair of transistors when added to the resistivity of said firstand second voltage controlled voltage sources which is substantiallyequal to a resistivity of a load across said first and second signaloutputs.
 13. The differential driver as recited in claim 12, whereinsaid load is a differential load.
 14. A method of driving a signal, saidmethod comprising: providing a switching module having a first andsecond voltage input, a signal input, and a signal output, said signalinput being connected to a plurality of switches in order to control anoperation of said switches, and said signal output being connected tosaid first and second voltage inputs via said plurality of switches;providing a first voltage controlled voltage source having a firstvoltage output having a low impedance; providing a second voltagecontrolled voltage source having a second voltage output having a lowimpedance; connecting said voltage output of said first voltagecontrolled voltage source to said first voltage input of said switchingmodule; and connecting said voltage output of said second voltagecontrolled voltage source to said second voltage input of said switchingmodule; wherein said steps of providing said first and second voltagecontrolled voltage sources comprise providing a first and second sourcefollower circuit at said first and second voltage outputs, respectively.15. The method as recited in claim 14, further comprising a step ofbiasing said first and second voltage controlled voltage source in orderto produce an output signal at said signal output having a voltage and acurrent within a predetermined range.
 16. The method as recited in claim14, wherein said step of providing said first and second voltagecontrolled voltage sources comprise providing a first source followercircuit comprising a transistor of a first type and said second sourcefollower comprising a transistor of a second type.
 17. The method asrecited in claim 16, wherein said step of providing said first andsecond voltage controlled voltage sources comprise providing a firstsource follower circuit comprising a transistor of a N-type and saidsecond source follower comprising a transistor of a P-type.
 18. Themethod as recited in claim 17, wherein said step of providing aswitching module comprises: providing a pair of PMOS transistorsconnected to said first voltage input, and a pair of NMOS transistorsconnected to said second voltage input; connecting a source of said PMOStransistors to a source of said N-type transistor of said first sourcefollower circuit; connecting a source of said NMOS transistors to asource of said P-type transistor of said second source follower circuit;connecting a drain of said PMOS transistors to a drain of said NMOStransistors; and connecting a gate of said NMOS and PMOS transistorseach to said signal input.
 19. The method as recited in claim 15,wherein said step of biasing said first and second voltage controlledvoltage source includes biasing said first and second voltage controlledvoltage source in order to produce an output signal at said signaloutput having a voltage and a current within an LVDS range.
 20. Themethod as recited in claim 18, wherein said step of biasing said firstand second voltage controlled voltage source includes biasing said firstand second voltage controlled voltage source in order to produce anoutput signal at said signal output having a voltage and a currentwithin an LVDS range.
 21. A differential driver comprising: a switchingmeans for switching a plurality of switches in order to produce a signaloutput based on an input signal, a first and second voltage input; afirst voltage controlled voltage source means for generating a first lowimpedance voltage output as said first voltage input to said switchingmeans; and a second voltage controlled voltage source means forgenerating a second low impedance voltage output as said second voltageinput to said switching means; wherein said first voltage controlledvoltage source means comprises a first source follower means forgenerating said first low impedance voltage output and said secondvoltage controlled voltage source comprises a second source followermeans for (generating said second low impedance voltage output.
 22. Thedifferential driver as recited in claim 21, further comprising: a biasmeans for biasing said first and second voltage controlled voltagesource means to produce said first and second low impedance voltageoutputs.
 23. The differential driver as recited in claim 22, whereinsaid bias means biases said first and second voltage controlled voltagesource means to produce said first and second low impedance voltageoutputs, such that said signal output of said switching means has acurrent and a voltage within a predetermined range.
 24. The differentialdriver as recited in claim 23, wherein said bias means biases said firstand second voltage controlled voltage source means to produce said firstand second low impedance voltage outputs, such that said signal outputof said switching means has a current and a voltage within a LVDS range.25. The differential drive as recited in claim 21, wherein saidswitching means includes transistor circuit means for switching acurrent from said first voltage controlled voltage source means to saidsignal output to said second voltage controlled voltage source means.26. The differential driver as recited in claim 21, wherein saidtransistor circuit means comprising a first group of transistorsconfigured to be switched ON when said input signal is positive, and asecond group of transistors configured to be switched ON when saidsecond signal is negative, said transistor circuit means for providing afirst output signal when said input signal is positive and a secondoutput signal when said second signal is negative.
 27. The differentialdriver as recited in claim 26, wherein said transistor circuit meansincludes a built-in terminal resistance means for improving outputsignal integrity.
 28. The differential driver as recited in claim 27,further comprising a differential load means for receiving said outputof said switching circuit, wherein said built-in terminal resistancemeans is based on a resistivity of each transistor pair and on aresistance of said differential load.
 29. The differential driver asrecited in claim 26, wherein said transistor circuit means includes atleast one transistor of a first type and at least on transistor of asaid second type.
 30. The differential driver as recited in claim 29,wherein said first type is a N-type transistor and said second type is aP-type transistor.
 31. The differential driver as recited in claim 28,wherein the resistivity of said first group of transistors when added tothe resistivity of said first and second voltage controlled voltagesources which is substantially equal to a resistivity of saiddifferential load, and the resistivity of said second pair oftransistors when added to the resistivity of said first and secondvoltage controlled voltage sources which is substantially equal to aresistivity of said differential load.
 32. The differential driver asrecited in claim 21, wherein said first source follower means includes afirst protective resistor means for protecting a transistor of saidfirst source follower means, and said second source follower meansincludes a second protective resistor means for protecting a transistorof said second source follower transistor.